Provide access to properties via PMF.getProperties()

The JDO spec says that PMF.getProperties is for returning non-modifiable properties. It doesn't make clear what one of those is. We could interpret is any persistence property and just provide access to them through this method

Description

The JDO spec says that PMF.getProperties is for returning non-modifiable properties. It doesn't make clear what one of those is. We could interpret is any persistence property and just provide access to them through this method